Ease of manufacturing. The Gen 4 frame dustcover is the same on both 9mm and .40 guns. The Gen 4 slide recoil guide hole is needed to match the Gen 4 frame dustcover. An identical recoil spring set-up is used to keep manufacturing streamlined. Why would you machine the slide recoil holes to take a different recoil rod assembly on the 9mm guns? If anything, install a lighter spring of the same physical design (they did) into the 9mm guns.

This is the same reason the Gen 3 guns in 9mm got the extra locking block pin that they never needed. Why have a .40/.357 production line seperate from the 9mm production line when the only difference is a $2 pin?

I have a Gen4 17. About 1000 rounds shot and only one failure to eject. That was from the first loaded mag using handloads. From that point on, everything I have tried has been just as one would expect from a Glock. Recoil is mild. Not too crazy about the accuracy when compaired to my P99 though. I may need to practice more.